He observed that probability assessments by humans differ from the calculable risks in nature and science. He believed our personal probabilities are what we think they are, not those predetermined by unseen natural forces. Similarly, to Keynes, and to proponents of personal probabilities, the value of a stock is what a group of sometimes irrational speculators is willing to pay for it. Kenneth Arrow went on to enshrine this valuation theory in his revolutionary model of security prices.

A contemporary of Keynes, the investor Bernard Baruch, is reputed to have quipped that “if economists are so smart, why aren’t they rich?” When Keynes died, his net worth, mostly from his investments, was about $30 million in today’s dollars.

Warren Buffett, the billionaire chairman of Berkshire Hathaway Inc., modeled his “value investing” after Keynes’s investment strategy, and once said the economist’s “brilliance as a practicing investor matched his brilliance in thought.”When Keynes died, the Financial Times reported: “Some surprise has been expressed about the large fortune left by Lord Keynes, yet he was one of the few economists with the practical ability to make money.”